Melt generation at very slow-spreading oceanic ridges: Constraints from geochemical and geophysical data

Abstract

We show that there is a strong and consistent correlation between geochemical and geophysical estimates of the amount of melt generated in the mantle beneath oceanic ridges. This correlation holds across all spreading rates and on scales down to the size of individual ridge segments. There is an abrupt decrease in the amount of melt generated at full spreading rates below 20 mm/a. Our observations are consistent with the conclusion that < 10% of the melt is frozen in the mantle before it reaches the crust and that serpentine probably represents only a small percentage of the material above the Moho. The melt is well mixed on a ridge segment scale, probably in high-level magma chambers, but the melts remain distinct between segments. The rare earth element concentrations of basalts from very slow-spreading ridges are higher than those from normal oceanic ridges, which is directly indicative of reduced mantle melting, and they show characteristic light rare earth element nerichment, interpreted as caused by a deep tail of small percentage wet melting. The decrease in melt production at rates below 20 mm/a points to the importance of conductive cooling inhibiting melting of the upwelling mantle at very slow-spreading centres.